A wide variety of digital systems communicate using signal wires. Integrated circuits or chips contain output buffers or drivers that drive an external wire such as a metal trace on a printed-circuit board or a cable connecting networked computers. Two basic techniques are used to pass digital information over external wires: single-wire and two-wire (differential) transmission.
The most common method is to simply transmit the signal over a single external wire. FIG. 1A highlights single-wire transmission. Chip 10 sends a signal to chip 12 over single external wire 18. Output driver 14 in chip 10 receives an internal signal from within chip 10 and drives a much higher current out to external wire 18. An input buffer or receiver 16 in chip 12 is connected to external wire 18. Receiver 16 determines the logical value of the signal on external wire 18 and amplifies and buffers the received signal to internal circuitry within chip 12.
FIG. 1B shows that the signal on the single wire is compared with a logic threshold. Input buffer receivers are designed to have a voltage midpoint or threshold V.sub.T that the input signal is compared with. The "comparison" with the threshold V.sub.T may be implicit in the design and technology of the input buffer rather than an explicit comparison of two voltages. Thus when the received signal is above V.sub.T a logical high is sensed, while a logical low is sensed when the received signal is below V.sub.T.
The actual voltage threshold can vary with the technology (process), power-supply voltage, and even the temperature. The ground levels of the two chips can differ. Noise can be coupled into the external wire or to the input buffer through power or ground supplies, or even from other internal circuitry. The noise can raise or lower the threshold significantly. When noise is sufficiently large or the threshold variation is extreme, the wrong logical value of the signal on the external wire can be detected. System failures can result.
The signal transmitted over the single wire may change rapidly. In higher-speed systems it may be desired to sense the signal before the transmitter has finished driving the external wire fully to the intended logic levels. Transmission through noisy environments may also be necessary.
Differential or two-wire transmission is a more robust signaling technique. FIG. 2A shows a differential signal passed between two chips. Rather than use a single wire for each signal, two wires 30, 32 together carry a single logical signal. The wires carry complementary signals: when wire 30 is driven low by driver 22, wire 32 is driven high by driver 24. When driver 22 drives wire 30 high, driver 24 drives wire 32 low. At steady-state after drivers 22, 24 have had enough time to charge or discharge capacitances on wires 30, 32, the logical states of the two wires 30, 32 are opposite.
Inverter 25 inverts an internal signal in chip 10 so that drivers 22, 24 always drive opposite signals to wires 30, 32. Receiver 26 in chip 12 receives both wires 30, 32, and compares their voltages. When the voltage on wire 30 is higher than the voltage on wire 32, a logical high is detected and output to internal circuitry in chip 12. Otherwise, a logical low is detected.
FIG. 2B shows waveforms of a differential pair of wires. Wire 30 has waveform 30', while wire 32 has waveform 32'. When 30' is higher in voltage than 32', such as at the beginning and end of FIG. 2B, a logical high is sensed. When 32' is higher than 30', such as in the middle, a logical low is detected.
Since the two wires 30, 32 are usually in close proximity to one another, and of the same length, any noise injected into one wire is also injected into the other wire. Thus external noise tends to cancel out. Noise from within chip 12, such as ground or power level variations, affects both wires 30, 32 equally. The relative voltages of the two wires, or voltage difference, is not affected by such common-mode noise. Enormous amounts of common-mode noise can be tolerated by differential signaling. There is no fixed threshold voltage that each of the two wires is compared to, as was true for single-wire sensing. Thus threshold variations are not problematic.
The transmitter chip 10 can have a different ground potential than receiver chip 12, since the ground shift affects both wires 30, 32 equally. Very small voltage differences between the two wires 30, 32 can be detected and amplified using current technology. Sensing is faster since smaller voltage swings can be used. Sometimes the voltage swings of the output drivers is purposely limited or clamped so that external capacitances are not full charged. This reduced power consumption and limits noise generated by the two wires. Radiation causing electromagnetic interference (EMI) is also reduced when voltage swings are limited.
FIG. 3 illustrates a prior-art signaling technique using differential-pairs or wires. Chip 10 transmits a multi-signal bus to chip 12 using differential pairs of wires 30, 32. One bit of the bus is transmitted over every two external wires 30, 32. Differential comparators 26 in chip 26 each receive a pair of input wires 30, 32, compare the two inputs, and generate a single bit that is output to internal bus 34.
The 6 bits of internal bus 34 require 12 external wires. Each bit of internal bus 12 was transmitted as a true and a complement signal over a pair of external wires. In general, differential transmission of an N-bit bus requires 2N external wires.
While such differential signaling techniques are more noise tolerant than single-wire signaling, costs are higher. The number of external wires is doubled. Bonding pads for external wires occupy a large area on many integrated circuit chips, increasing cost. Leads on the IC packages are also limited. Increasing the number of external wires connected to a chip, or its lead count, is expensive since larger packages may be required.
What is desired is a differential signaling technique that uses fewer wires. It is desired to reduce the number of external wires needed when transmitting digital signals. It is desired to use differential-comparator receivers that are insensitive to common-mode noise. Differential sensing is desired for noise immunity, while differential transmission is desired so that voltage swings and noise generation can be limited. A compressed differential-signaling technique is desired that uses fewer wires. It is desired to use fewer than 2N external wires while still benefiting from differential sensing.